I heard this was coming in a few weeks, but it is here now....

This is a free download, but note that you have to have your original CD-Key for this to work.... and that the version that supports custom maps is multi-player only and doesn't ship with any of the original maps... the dedicated server program is included though!!

[Update: Note sure about that 'doesn't ship with maps' comment I made... I read that in an article on the web, but the maps appeared to be there when I played... perhaps the new maps added in Halo PC are missing... but I can't be sure]